The present invention relates to a composition for coating on leather substrates, which may be cured in situ by means of radiation and to a method for applying the composition to leather.
Leathers, and particularly leathers for use in shoe uppers, are frequently coated with polymers to enhance properties, such as wet and dry scuff and abrasion resistance, water resistance, gloss, and the like.
The range of polymer compositions suitable for use in such applications has hitherto been severely limited. On the one hand it is difficult to formulate compositions, which will confer these properties on leather while retaining desirable "feel", appearance, air permeability, flexibility and other desirable characteristics of leather. On the other hand, the methods of obtaining tough polymer coatings usually used in other industries generally involve cross-linking by heat and are inapplicable to leather by virtue of leather's inability to withstand heat without detrimental effects.
In fact, in the past, only nitrocellulose and P.V.C. coatings have met with any commercial success for imparting toughness and other of the desirable properties to which reference has been made above. Neither nitrocellulose nor P.V.C. coatings impart all of the desirable characteristics in the desired degree, but to date have been the most satisfactory.
Moreover application of polymer coatings such as previously used is time consuming, labour intensive and costly. For example nitrocellulose is sprayed from solvents solution onto individual hides. Four separate coatings are usually necessary to build up the thickness required to confer desirable properties, and drying is required between each spraying operation.
It is well known that polymers may be cross-linked by radiation. However early attempts to apply gamma radiation for this purpose to polymer coatings on leather substrates were unsatisfactory because of the detrimental effect of gamma radiation on the leather at the dose rates required to achieve sufficient cross-linking.
Moreover, as a general rule polymers, which tend to cure rapidly under the influence of radiation, tend not to have a commercially practicable shelf life.
To be commercially useful as a coating for leather, a radiation curable composition should have a stable shelf life, a viscosity enabling convenient application to the substrate or be capable of adjustment to a convenient viscosity, be capable of rapid cure by radiation under conditions not detrimental to leather, in addition to forming films having at least the attributes of previously available nitrocellulose compositions.
Thus, to date it has not proved possible to produce a radiation curable surface coating for leather, which would perform satisfactorily in comparison with traditional nitrocellulose coatings, and which also could be applied so that, in the finished product, it would be economically competitive with nitrocellulose coatings.